1. Field of the Invention
The present invention relates to a holding sealing material, an exhaust gas purifying apparatus, and a method for manufacturing an exhaust gas purifying apparatus.
2. Discussion of the Background
Particulate matters (hereinafter, also referred to as PMs) are contained in exhaust gases discharged from internal combustion engines such as diesel engines, and in recent years, there has arisen a serious problem as these PMs are harmful to the environment and the human bodies. Moreover, since exhaust gases also contain toxic gas components such as CO, HC, and NOx, there have been growing concerns about influences of these toxic gas components on the environment and the human bodies.
In view of these, as an exhaust gas purifying apparatus for collecting PMs in exhaust gases and for purifying the toxic gas components, various exhaust gas purifying apparatuses have been proposed. Each of the exhaust gas purifying apparatuses is configured by: an exhaust gas treating body made of porous ceramics, such as silicon carbide and cordierite; a casing configured to house the exhaust gas treating body; and a holding sealing material made of an inorganic fiber aggregated body that is disposed between the exhaust gas treating body and the casing. This holding sealing material is installed mainly for purposes of preventing the exhaust gas treating body from being damaged upon contact with the casing that covers its periphery due to vibrations and impacts caused by traveling or the like of an automobile and for preventing exhaust gases from leaking between the exhaust gas treating body and the casing (for example, see WO 02/38922).
In recent years, a technique referred to as a urea SCR (Selective Catalyst Reaction) has been examined as a method for converting NOx in exhaust gases.
The urea SCR system sprays urea water in an exhaust gas purifying apparatus, and ammonia is generated by thermal decomposition of urea so that NOx undergoes a reduction reaction by ammonia.
FIG. 1 is a cross-sectional view that schematically shows a state where urea water is sprayed in an exhaust gas purifying apparatus having a conventional holding sealing material.
An exhaust gas purifying apparatus 1020, shown in FIG. 1, has a structure in which a holding sealing material 1010 is wound around an exhaust gas treating body 30 made of a honeycomb structure (a catalyst supporting carrier), and this is disposed inside a casing 40, and a urea spray opening 100 configured to spray urea water 110 is formed on a portion near the exhaust gas treating body 30 (on the upstream side of exhaust gases) through which exhaust gases are allowed to flow into the exhaust gas treating body 30.
In the urea SCR system, the urea water is sprayed toward the exhaust gas treating body. Then, urea contained in the urea water is thermally decomposed by heat of the exhaust gases to generate ammonia so that ammonia and NOx in the exhaust gases undergo a reduction reaction to form N2 by a function of a catalyst supported on the exhaust gas treating body; thus, NOx is converted. Moreover, the exhaust gases in which NOx has been converted are discharged from the downstream side of the exhaust gas treating body.
The contents of WO 02/38922 are incorporated herein by reference in their entirety.